Surface acoustic wave (SAW) devices are widely employed for resonators, bandpass filters, duplexers, and transformers in RF and microwave communication devices, such as mobile telephones.
FIG. 1 is a schematic illustration of a SAW device 100. SAW device 100 includes first and second interdigitated transducer (IDT) electrode pairs 140 and 150 formed on a piezo-electric substrate. A surface acoustic wave may be excited by signal generator 10 applying an electrical signal across IDT electrode pair 140. A load 20 is provided as shown. Electrical signals are correspondingly generated across IDT electrode pair 150 from the passing surface acoustic waves.
FIG. 2 illustrates a cross-section of a portion of an example of a SAW device 200. SAW device 200 includes a base substrate 230 and first and second electrodes 242 and 244 disposed on a top of base substrate 230. Base substrate 230 is made of a piezo-electric material, such as lithium tantalite (LiTaO3—hereinafter simply referred to as LT) or lithium niobate (LiNbO3—hereinafter simply referred to as LN), for example. First and second electrodes 242 and 244 comprise an electrically conductive material such as gold or aluminum, and may be part of an IDT electrode pair as illustrated in FIG. 2. Notably, first and second electrodes 242 and 244 represent only one pair of a plurality of electrodes 140 or 150 forming an IDT electrode pair shown in FIG. 1. The operation of SAW devices is well known to those skilled in the art, and therefore will not be reiterated here.
Higher performance is continuously being sought for SAW devices as the performance requirements of the devices in which they are embedded become more demanding. For example, in recent years it has been desired to provide SAW devices with enhanced temperature stability, because changes in frequency can altar the frequency characteristics of a SAW device. In particular, for a SAW filter, changes in temperature may altar the pass-band frequency range of the SAW filter.
As is known, piezo-electric materials having a large electromechanical coupling coefficient are advantageous for realizing broad filter characteristics. LT and LN are examples of piezo-electric materials having a large electromechanical coupling coefficient. However, the temperature stability of LT and LN leaves something to be desired. There seems to be a general tendency of incompatible characteristics such that piezo-electric materials having large electromechanical coupling coefficients such as LT and LN have comparatively poor temperature stability, while piezo-electric materials having good temperature stability, such as quartz crystal, have comparatively small electromechanical coupling coefficients.
Accordingly, SAW devices have been developed which employ hybrid or composite substrate structures.
FIG. 3 illustrates a cross-section of a portion of an example of a SAW device 300 having a so-called composite substrate 302. In particular, SAW device 300 includes composite substrate 302 comprising a base substrate 310 of a first material, and a piezo-electric material layer 330 disposed on and immediately adjacent to base substrate 310. SAW device further includes first and second electrodes 342 and 344 disposed on a top of piezo-electric material layer 330.
Piezo-electric material layer 330 is made of a piezo-electric material, and beneficially a piezo-electric material having a large electromechanical coupling coefficient, such as LT or LN. Base layer 310 is made of a different material than piezo-electric material layer 330, and beneficially is a material having a relatively low-coefficient-of-thermal-expansion—at least lower than that of piezo-electric material layer 330. First and second electrodes 342 and 344 comprise an electrically conductive material such as gold or aluminum, and may be part of a pair of interdigitated transducers by which a signal is propagated from an input transducer to an output transducer.
Overall, when compared to SAW 200, SAW 300 may exhibit some tangible benefits such as improved thermal performance, some temperature compensation, and an improved quality factor (“Q”) below its fundamental operating frequency.
Unfortunately, however, in general a SAW device having the general structure of SAW device 300 with its composite substrate 302 is typically prone to exhibiting spurious responses or “rattles” at frequencies above its series resonance frequency, Fs.
For example, FIG. 4 plots an example of the simulated global admittance frequency response for an example of SAW device 300 where piezo-electric material layer 330 comprises LT and has a thickness of 20 μm, and base substrate 310 comprises silicon (Si) having a thickness of 20 μm or more. As seen in FIG. 4, the admittance frequency response has a peak 410 at a series resonance frequency, Fs, of about 759 MHz, which is the desired response. However, the admittance frequency response also has a number of spurious responses or “rattles” 420 at frequencies above Fs.
These spurious responses or “rattles” are undesirable. For example, if the SAW device is a bandpass filter, then any undesired signals which fall on a frequency of one of the spurious responses may pass through the SAW filter without the desired level of attenuation.
What is needed, therefore, are SAW devices which may exhibit a large electromechanical coupling coefficient, good temperature stability, and low levels of spurious responses or “rattles.”